1. Field of the Invention
The present invention relates to a method for maintaining uplink timing synchronization (sync) in an Orthogonal Frequency Division Multiplexing (OFDM) system and a User Equipment (UE) apparatus for the same.
2. Description of the Related Art
The mobile communication scheme can be classified into Time Division Multiplexing (TDM), Code Division Multiplexing (CDM) and Orthogonal Frequency Multiplexing (OFM) schemes according to a multiplexing method. The CDM scheme is most popularly used in the current mobile communication system, and can be subdivided into a synchronous and an asynchronous CDM scheme. Since the CDM scheme basically uses codes, it tends to suffer from a lack of resources due to a limit of orthogonal code resources. Accordingly, an OFDM scheme has emerged as an alternative to the CDM scheme.
The OFDM scheme is for transmitting data using multiple carriers, and is a type of Multi-Carrier Modulation (MCM) scheme that converts a serial input symbol stream into parallel streams, and modulates each of the parallel streams with a plurality of orthogonal sub-carriers, i.e. sub-carrier channels, before transmission. The OFDM scheme is similar to the conventional Frequency Division Multiplexing (FDM) scheme, but it maintains orthogonality between multiple sub-carriers during transmission and overlaps frequency spectra. Therefore, the OFDM scheme has high frequency efficiency, is robust against frequency selective fading and multi-path fading, and can reduce Inter-Symbol Interference (ISI) with use of a guard interval. In addition, the OFDM scheme enables simple design of a hardware equalizer and is robust against impulse noises, so it can obtain the optimal transmission efficiency during high-speed data transmission.
A Long Term Evolution (LTE) system employing the OFDM scheme is now under discussion in 3rd Generation Partnership Project (3GPP) as the next generation mobile communication system that will replace Universal Mobile Telecommunication System (UMTS), which is the 3rd generation mobile communication standard.
FIGS. 1A and 1B illustrate examples of a wireless mobile communication system to which reference will be made by the present invention, particularly illustrating examples of a 3GPP LTE system.
Referring to FIG. 1A, a UE 11 indicates a terminal for the 3GPP LTE system, and an Evolved Radio Access Network (E-RAN) 14, a radio base station device directly participating in communication with a terminal in the existing 3GPP system serves as a Node B for managing cells, and also serves as a Radio Network Controller (RNC) that controls a plurality of Node Bs and radio resources. In the E-RAN 14, an Evolved Node B (E-NB) 12 and an Evolved RNC (E-RNC) 13 can be separately implemented in the physically different nodes, or can be merged in a single node, in the manner of the existing 3GPP system. Although in the following description the E-NB 12 and the E-RNC 13 are physically merged in a single node of the E-RAN 14, the same can be applied to when the E-RNC 13 is separately implemented in the physically different node.
An Evolved Core Network (E-CN) 15 is a node provided by merging functions of a Serving GPRS Support Node (SGSN) and a Gateway GPRS Support Node (GGSN) in the existing 3GPP system into one function. The E-CN 15, interposed between a Packet Data Network (PDN) 16 and the E-RAN 14, serves as a gateway for allocating an Internet Protocol (IP) address to the UE 11 and connecting the UE 11 to the PDN 16. Definitions and functions of the SGSN and the GGSN follow the 3GPP standard, and a detailed description thereof will be omitted herein.
Referring to FIG. 1B, an Evolved UMTS Radio Access Network (E-RAN) 110 is simplified to a 2-node configuration of Evolved Node Bs (E-NBs) 120, 122, 124, 126 and 128, and anchor nodes 130 and 132. A UE 101, or a terminal, accesses an IP network by the E-RAN 110. The E-NBs 120 to 128 correspond to the existing Node Bs in the UMTS system, and are connected to the UE 101 over a wireless channel. Unlike the existing Node Bs, the E-NBs 120 to 128 perform more complex functions. In LTE, because all user traffics, including real-time services such as Voice over IP (VoIP), are serviced over a shared channel, there is a need for devices for gathering status information of UEs and performing scheduling depending thereon, and the E-NBs 120 to 128 manage the devices.
Generally, one E-NB controls a plurality of cells. In addition, the E-NB performs Adaptive Modulation & Coding (AMC) that determines a modulation scheme and a channel coding rate according to channel status of a UE. Similar to High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA) and Enhanced Dedicated CHannel (E-DCH) of UMTS, even in LTE, Hybrid Automatic Repeat reQuest (HARQ) is performed between the E-NB 120 to 128 and the UE 101. However, because LTE cannot meet various Quality of Service (QoS) requirements only with HARQ, Outer-ARQ in an upper layer can be performed between the UE 101 and the E-NBs 120 to 128. HARQ, as is well known, refers to a technique for soft-combining previously received data with retransmitted data without discarding the previously received data, thereby increasing a reception success rate. In high-speed packet communication, such as HSDPA and EDCH, the HARQ technique is used to increase transmission efficiency. It is expected that to realize a data rate of a maximum of 100 Mbps, LTE will use OFDM as a wireless access technology in a 20-MHz bandwidth.
FIG. 2 illustrates an uplink timing synchronization procedure in a 3GPP LTE system to which OFDM is applied.
Referring to FIG. 2, a first UE (UE1) is located near an E-NB, and a second UE (UE2) is located far from the E-NB. T_pro1 indicates a propagation delay time in wireless transmission up to the UE1, and T_pro2 indicates a propagation delay time in wireless transmission up to the UE2. Because the UE1 is located nearer to the E-NB compared to the UE2, it has less propagation delay time. In FIG. 2, T_pro1 is 0.33 us, and T_pro2 is 3.33 us.
In one cell (indicated by a circle in FIG. 2) of the E-NB, when the UE1 and the UE2 are powered on or are in an idle mode, uplink timing synchronization of the UE1, the UE2 and of UEs in the cell, detected by the E-NB, are not matched to each other. Reference numeral 201 indicates timing synchronization for uplink transmission of an OFDM symbol of the UE1, and reference numeral 202 indicates timing synchronization for uplink transmission of an OFDM symbol of the UE2. When propagation delay times of uplink transmission of the UE1 and the UE2 are considered, timings at the E-NB receiving the uplink OFDM symbols are shown by reference numerals 211, 212 and 213. That is, the uplink symbol 201 of the UE1 is received at the E-NB with a propagation delay time in the timing 212, and the uplink symbol 202 of the UE2 is received at the E-NB with a propagation delay time in the timing 213.
Since uplink timing synchronizations for the UE1 and the UE2 have not been acquired (matched) yet for the timings 212 and 213, start timing 211 in which the E-NB receives and decodes an uplink OFDM symbol, timing 212 in which the E-NB receives an OFDM symbol from the UE1, and timing 213 in which the E-NB receives an OFDM symbol from the UE2 are different from each other. Therefore, the uplink symbols transmitted from the UE1 and the UE2 serve as interference components to each other, as they have no orthogonality, and the E-NB may not successfully decode the uplink symbols 201 and 202 transmitted from the UE1 and the UE2, due to the interference and the discrepancy between the start timing 211 and the reception timings 212 and 213 of uplink symbols.
Therefore, the E-NB matches uplink symbol reception timings of the UE1 and the UE2 through the uplink timing synchronization procedure. After completion of the uplink timing synchronization procedure, the E-NB can match the start timing 221 in which it receives and decodes uplink OFDM symbols, the timing 222 in which it receives an uplink OFDM symbol from the UE1, and the timing 223 in which it receives an uplink OFDM symbol from the UE2. After matching the timings, the E-NB can maintain orthogonality between the uplink symbols transmitted from the UE1 and the UE2, and thus can successfully decode the uplink symbols 201 and 202 transmitted from the UE1 and the UE2.
FIG. 3 illustrates an example of an uplink timing synchronization procedure.
In step 311, a UE 301 generates a preamble code to be used in the uplink timing synchronization procedure. If the UE 301 is constructed such that multiple preamble codes can be used in the uplink timing synchronization procedure, the UE 301 generates one of the multiple preamble codes. The ‘preamble code’ is a type of code sequence agreed upon between the UE 301 and an E-NB 302, and the UE 301 transmits the preamble code over the uplink using radio resources allocated by the E-NB 302 in step 321 (UL SYNC REQ). Upon receipt of the preamble code, the E-NB 302 calculates a correlation between the preamble code and candidate preamble codes available for uplink timing synchronization during a sliding window having a certain constant interval, to find the timing and preamble code indicating the highest correlation. In addition, the E-NB 302 calculates a difference between the then-reception timing and the timing in which it should actually have received the preamble code, and provides in step 322 the UE 301 with an IDentifier (ID) of the found preamble code and information on the uplink timing difference using a response message (UL SYNC RES). In step 331, the UE 301 changes and updates the uplink transmission timing using the information on the uplink timing difference, received through the response message. From this time one, uplink signaling and data transmission is achieved using the changed and updated uplink timing.
Steps 341, 342 and 343 indicate a process of re-performing the uplink timing synchronization procedure in steps 311, 321 and 322 to recheck the changed and updated timing, and can be omitted.
The uplink timing synchronization procedure shown in FIG. 3 should be periodically performed because the UE in the mobile communication system continuously moves, and thus the distance difference between the UE and the E-NB may change over time. When the periodic uplink timing synchronization procedure is performed, the UE periodically generates a preamble code used for the uplink timing synchronization procedure and transmits the preamble code to the E-NB over the uplink, and the E-NB should find an uplink timing difference by receiving and decrypting the periodic uplink preamble code, and provide the uplink timing difference information to the UE over the downlink. Therefore, overhead of the uplink signaling/downlink signaling occurs, causing inefficient use of radio resources.